Hob for gear-cuttings.



E. J. LEES.

HOB FOR GEAR GUTTINGS.

APPLICATION FILED APB.19,1907.

Patented June 15, 1909.,

Lav enfort mg I $1.04! 3 m UNITED STATES PATEN T OFFICE.-

ERNEST J. LEES, OF CLEVELAND, OHIO, ASSIGNOR TO THE .GRA'NT-LEES MACHINE COMPANY,

- OF CLEVELAND, OHIO, A CORPORATION OF OHIO.

' HOB FOR GEAR-GUTTILIGS.

Specification of Letters Patent.

Patented June 15. 1909.

Application filed. April 19, 1907. Serial No. 369,057.

The object of the present invention is to provide an improved hob for the generating of gears.

The particular urpose is to provide aform of hob which-will be capable of cutting accurate teeth on the gears so that they will mesh closely and at the. same time have pro er clearance s ace between the teeth suc i as should be 'ad in order to take care of the dirt which always accumulates at such points.

An embodiment of my invention is illustrated in the accompanying drawings in which- Figure '1 is a lon itudinal section of one of my improved hobs showing the generating teeth to be of the true involute basic form commonly called the rack. Fi 2 is an end elevation of Fig. 1. Fig. 3 is a diagrammatic illustration of the manner of operation compared with the operation of other forms of hobs.

In the cutting of gear teeth, there are, of

i course, many methods adopted, but it has entails, as a conseque ice,

been found that cutting them with a hob is preferable because of the regularity of spacing of the teeth and the rapidity and ease with which the cutting is accom lished. The cutting operation itself causes t e gear blank'to be fed in the roper manner and this, while being an akfvantage, obviously a eculiar movement of the cutting edges of tiiehob through the blank. When hob cutters of the ordinary contour, shaped to cut clearance space, followthe line of travel through the blank the result is that the gear teeth take a contour such that they fit imperfectly. This will be made clear by referring to the diagram shown in Fig. 3. In this case we will let the full line 1 -1 re resent the outline of the gear teeth A ascut y my im roved form of hob B the cutting edges of W1l0l1 will be represented by the full; line 2-2, Now suppose the dimension line 33 designates that depth of the cutter which cuts out what is commonly designated as the working depth of the gears, that is the depth to which the meshing teeth will engage each other.

It is sometimes the practice to have the hob cutter terminate in a cutting edge along the line 44. and, while the curve of the teeth is properly =formed to insure close fitting, nevertheless no clearance space is provided by. such a hob, and the intermeshing teeth will ground. This may make no diiieronce when the fit is absolutely accurate and when there is no dirt or deposit in the space between the teeth, but when such deposit ocours, obviously the meshing of the gears will cease to be close or correct. It has therefore been, proposed, in order to avoid this grounding of the teeth, to extend the hob cutters to a greater height than is necessary to provide for cutting out the gear teeth to the ordinary working depth and thereby cause a clearance space to be formed. This proposition has merely involved the extending of the tapered straight side lines of the cutter so'that the top edge of the cutter will be advanced a slight distance beyond the line 4. This would produce a top cutting edge along the dotted line 55 shown in Fig. 3, the sharp corners of which would follow along the path shown in the dotted lines 6. That portion of the cutter between the lines 4-a and 55 would make a clearancespace at the base or" the teeth all right but the travel of the corners 55 alon the line 66 destroys the proper contour of the teeth so that they no longer fit as they should.

It so happens that the use of hobs in the cutting of gears has been mainly confined to worm gears where this defective feature was not very apparent or of importance. 011 ordinary gears, however, it is a matter of great consequence and it is to remedy this that I have devised the hob having the form shown in Figs..1 and 2 and the outline of whose teeth is shown to be of the trueinvolute basic form commonly called the rack, following the line 22 of Fig. 8.

It will be observed t rat in this hob I extend the cutter to a height to enable it to cut however, I rovide with beveled or reduced corners whic may beeither curved or plane, according to the accuracy of the Work desired and the limit of cost on the manufacture.

With the corners of the clearance cutting ortion of the hob s0 rounded or beveled, as or example Fig. 3, it will be seen that the path of the cutter through the gear blank will be the same as usual, to the depth 33, thus forming'the contour of the gear teeth alon the proper lines so far as their working depti is concerned, while the ath of the clearance cutting portion will fa 1 within the lines cut by that portion of the line below the line 44. v

The precise angle .at which to give the curve or bevel at the outer corners of the cutter will dependupon the individual circumstances in. the case, and it is therefore not thought necessary to note any particular measurements, but it is necessary, in actual practice, that the reduction of the corners be sufiicient to keep them within the lines followed by that portion of the cutter below or behind the line 44 which marks the outer limit of that portion of the hob which determines the working depth of the gear teeth. I

shape of the flanks of the teeth being cut, but 4 etween the teeth. 7

2. A bobbing-cutter for generating gearteeth, comprising cutting teeth of involute rack tooth-shape in cross section but extended at the point .to rovide clearance in the gear being out, said extended portions being reduced in width so that their cuttin edges will not act on the flanks of the teetfi provide clearance at the bottom of the space being cut, to affect their correct shape but will provide clearance at the bottom of the space between the teeth.

In testimony whereof, I hereunto aflix my signature in the presence oftwo witnesses.

ERNEST J. LEES. Witnesses JoHN A. ALBURN, H. R. SULLIVAN. 

